1. Field of the Invention
The present invention relates to a method of efficiently decontaminating a medium which contains polychiloinated biphenyls (hereinafter abbreviated as PCBs) or polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (hereinafter abbreviated as dioxins), such as soil or a lake or marsh contaminated with PCBs or dioxins, through utilization of plants' capability of absorbing, or absorbing and decomposing, PCBs or dioxins.
2. Description of the Related Art
Since PCBs are chemically very stable and exhibit excellent electrical insulating properties, they have been widely used in, for example, insulating oils of capacitors or transformers, thermal media, machine oils, and pressure-sensitive paper.
Since recognition of biological toxicity of PCBs (for example, in the Kanemi Oil case in Japan), production of PCBs has been regulated. However, contamination with PCBs has spread globally, and the effect of PCBs has lasted over a long period of time.
Recent reports have pointed out the toxicity of PCBs as endocrine disrupters and that even a very small amount of PCBs has an adverse effect on the environment.
PCB decomposition processes approved in Japan under the Waste Disposal and Public Cleansing Law are an incineration process, a base-catalyzed decomposition (BCD) process, a chemical extraction-decomposition process (DMI/NaOH process), a potassium tertiary butoxide process (t-BuOK process), a catalitic hydrogenation-dechlorination process (+t-BuOK process), and a supercritical water oxidation process (SCWO process).
Such chemical or physical decomposition processes exhibit relatively high decomposition efficiency, but conventionally involve high processing cost. Also, these processes are suited for decontaminating a contaminated medium of high contamination concentration, but are not suited for practical decontaminating of a large-scale, contaminated medium of low contamination concentration.
Meanwhile, dioxins, which are very toxic, are chemically relatively stable and thus are not easily decomposed. Since dioxins are soluble in oils and fats, once food contaminated with dioxins is taken into the human body, dioxins are absorbed by the organs. Since ingested dioxins are hardly egested, dioxins reportedly not only keep accumulating within the human body but are also transmitted from a mother to a fetus via the womb.
In recent years, as the action of dioxins as endocrine disrupters has become clear, the effect of dioxins on future generations has become of serious concern.
Dioxins are generated mainly in an incineration process at a temperature of about 300-850.degree. C., as in a refuse incineration furnace, and are released into the atmosphere in the form of exhaust gas. Thus-released dioxins are primarily accumulated in, for example, soil, lakes and marshes, and rivers. Since measures against dioxin sources have begun to work, fresh generation of dioxins tends to be suppressed. However, dioxins in the soil have been left accumulated over a long period of time without deterioration or decomposition, so that the soil contaminated with dioxins has become a secondary source of dioxins.
Incineration at high temperature is the most effective process for decomposing dioxins. This incineration process is not suited for treating a large amount of contaminated media and involves considerable treatment. According to a conventional method for disposing of, for example, contaminated soil, the contaminated soil is excavated and relocated to a disposal yard, where the contaminated soil is filled into an excavated hole and covered with sealing soil. This method is conventionally employed, since a relatively large amount of contaminated media can be disposed of.
This filling-up method is a tentative one--a medium contaminated with dioxins is merely moved from one location to another. The relocated, contaminated medium may raise another environmental contamination problem in the new location. Efficient, perpetual measures to treat dioxins have not been achieved.